Here is a good example of what some mild bowl cleanup, Chamber polishing, and gasket matching heads and intake bowl work on the exhaust and a good valve job with 30 degree backcut on the valves did for LT4 heads.
Intake manifold gasket
GM 12367777 LT4 gasket was used can also be used with ported lt1 heads and manifold.
Gasket port dimensions: 1.245" X 2.285". Center divider width 7/32" top width is 15/64" bottom widthof gasket ~ 1/4"'
LT4 Port Exit Dimensions: A-2.12" x 1.20"
Lt4 intake manifold and lt1 have same port dimension resulting in a mismatch with the larger LT4 Cylinder head Port openings at the top as you can see the LT4 manifold has more meat at the top for porting this mismatch.
Resuts near stock LT4: Rear wheel horse power increased by 22 and peak torque increased 15 ft lbs after the port work. However, torque over 5500 RPM increase about 20 ft/lbs. The engine idled exactly as before with no change in drivability.
Mez's LT4 Head Porting
Smaller LT1 intake gasket for stock or near stock heads
LT1 Port Exit Dimensions: A-2.06", B-1.15"
LT1 part # GM 12524653
LT1-LT4 Fel Pro FPP-1284 Steel Core Laminate/ Printoseal, 2.04 in. x 1.25 in. Port, .060 in. Thick
Felpro 1284 -- 0.075" thick, 2.071" x 1.275" ports (I.D.)
Blue Printoseal around ports Performance and ported heads.
Felpro 95580 -- 0.064" thick, 2.064" x 1.257" ports (I.D.)
Better for stock ports steel grey laminate.
MR Gasket LT1 Performance intake manifold gaskets Part #135G
Notes: LT1 port; port dimensions W-1.24" x H-2.06"; 0.0625" thick
Also need to consider head gaskets
Stock replacement for the LT1:
Fel Pro Permatorque Blue Part Number: 9966PT Cost $22ea
Victor/Reinz Part # 5922 Cost $29ea
This is the .026" thick MR Gasket Head Gasket.
I think I'll go with the medium Priced Fel Pro 1094's Stainless steel shim no brinelling of aluminum heads. Cost $29 ea
The High performance steel core laminate Fel-Pros are Part # 1074 Cost $49ea
Fel Pro also makes a MLS gasket similar to Cometics MLS Permatorque Blue 1144-071 $127ea